Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

Mark the incorrect statement in Kjeldahl's method of estimation of nitrogen (a) copper sulphate or mercury acts as a catalyst (b) nitrogen is quantitatively decomposed to give ammonium sulphate (c) nitrogen gas is collected over caustic potash solution (d) potassium sulphate is used as boiling point elevator of \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

Statement (c) is incorrect because nitrogen is not collected over caustic potash.

Step 1: Understanding Kjeldahl's Method

Kjeldahl's method is employed to determine the nitrogen content in organic compounds. The process involves converting nitrogen in the organic compound into ammonia, which is then estimated.

Step 2: Considering Copper Sulphate or Mercury as Catalysts

In the Kjeldahl method, a catalyst like copper sulphate or mercury is added to speed up the digestion process, which is true.

Step 3: Looking at Nitrogen Decomposition

During digestion in Kjeldahl's method, nitrogen in the organic sample is converted to ammonium sulphate, not decomposed into gaseous forms. Thus, the statement that nitrogen is decomposed to give ammonium sulphate is incorrect.

Step 4: Nitrogen Gas Collection Over Caustic Potash

Nitrogen gas is not collected over caustic potash in Kjeldahl's method. As nitrogen is usually converted into ammonia in the process rather than remaining as molecular nitrogen gas, this statement can also be incorrect.

Step 5: Potassium Sulphate's Role

Potassium sulphate is used as a boiling point elevator in the Kjeldahl method to raise the boiling point of the sulfuric acid used during the digestion, making this statement true.

Key Concepts

Nitrogen Estimation

In the realm of chemistry, particularly when it comes to analyzing organic compounds, the estimation of nitrogen content is a fundamental task. Kjeldahl's method is a well-established technique for this purpose. The primary goal of this method is to determine the amount of nitrogen present in organic compounds by converting it into a measurable form. During the procedure, the nitrogen in the compound is converted into ammonia through a series of reactions. These reactions predominantly happen in two stages: digestion and distillation.

First, the compound is digested with concentrated sulfuric acid, transforming all organic nitrogen into ammonium ions. In the subsequent step, the ammonium ions are then converted to ammonia gas by adding a strong base like sodium hydroxide. The ammonia is distilled over into a known volume of standard acid, allowing its quantification. By calculating the amount of ammonia produced, the original nitrogen content can be determined.

This method is particularly valued for its accuracy and has been widely used in agricultural and food industries to evaluate the protein content of various products.

Catalysts in Digestion

Catalysts are crucial in the digestion stage of Kjeldahl's method as they significantly enhance the efficiency of the reaction. During the digestion process, the organic material in the sample is broken down in the presence of concentrated sulfuric acid. Here, the role of catalysts is vital to make the process quicker and more effective.

Common catalysts used in this context include copper sulfate and mercury. These compounds help to accelerate the oxidation of the organic material, which might otherwise proceed too slowly to be practical in a laboratory setting. The catalyst facilitates the complete digestion of the sample by ensuring that all the organic nitrogen is fully converted to ammonium ions.

It's important to note that the choice of catalyst might vary depending on the specifics of the sample and environmental considerations. For instance, while mercury is a traditional choice, its use might be limited due to environmental concerns, prompting a preference for alternatives like copper sulfate.

Ammonium Sulphate Formation

Ammonium sulfate formation is at the heart of the Kjeldahl method during the digestion phase. When the sample is heated with concentrated sulfuric acid, the organic nitrogen in the sample is transformed into ammonium ions. This results from the acid breaking down the organic compounds and integrating the nitrogen into its structure.

After digestion, the solution primarily contains ammonium sulfate as a product along with the excess sulfuric acid. This transformation is crucial because it converts the nitrogen into a stable and measurable form, facilitating its quantification. By converting the nitrogen into ammonium ions, any potential interference from other nitrogen-containing compounds is minimized.

These ammonium ions are then turned into ammonia gas during the distillation phase, where they can be measured effectively. The formation of ammonium sulfate is essential as it ensures that nitrogen is anchored in a form that is amenable to accurate analytical procedures.